This invention relates to new and improved materials and devices for improved selective absorption of light energy for use in photothermal applications. Such devices are generally in the nature of a layered composite of materials which, as a whole, absorb sunlight and convert it to heat. A selective absorber for photothermal applications should exhibit high absorption (i.e., low reflectivity) in the terrestrial solar spectrum (TSS), wavelengths of from about 350 nm to about 2300 nm. This is desirable because the terrestrial solar spectrum comprises about 98% of the solar power which typically reaches the surface of the earth and is therefore generally most usable for generating heat. The device should also exhibit low emissivity (i.e. high reflectivity) in the thermal infra-red (TIR) portion of the spectrum, generally much above 2300 nm (for example centered around 7000 nm for an operation of 130.degree. C.), because these are the wavelengths at which bodies tend to radiate heat.
A number of prior art compositions and devices are currently in use as absorbers of solar energy. Among them are the materials like black paint. These materials are solar-nonselective. While exhibiting acceptable absorption in the TSS, these materials exhibit low reflectivity in the TIR. Hence, these materials are inefficient selective absorbers because the low reflectivity in the TIR results in high emissivity in the TIR. Such a characteristic, as noted above, results in emission of power as heat and thus makes these substances far less desirable for photothermal applications.
Another type of prior art material used in solar absorbers is aluminum with a nickel anodized coating. The aluminum provides reflectance in the infra-red regions which is desirable. The nickel anodizing coats the aluminum resulting in a black colored oxide over the aluminum which provides absorption and the rough texture provides some antireflection. The biggest problem with this approach is that an aluminum substrate must be used. Hence, other forms of substrates cannot be used without first applying a layer of aluminum thereto. This, of course, adds extra steps and therefore extra time and expense to the manufacturing process.
A third type of prior art material is referred to as black chrome. This type of material is generally composed of nickel electrolyzed in a chrome bath. The surface is coated with chromium and chromium oxide particles which provide both antireflection and absorption. This approach suffers from the same deficiency as the nickel anodized aluminum selective absorbers in that they cannot be applied to a non-conductive substrate without the interposition of a conductive layer.
Another prior art material in use as a selective absorber of light energy is black cobalt, Co.sub.3 O.sub.4. This is produced by electrolyzing a substrate in a cobalt bath. Here again, a conductive layer is required if a non-conductive substrate is to be employed.
Two other prior art materials are seemingly well suited to use as selective absorbers but they share a significant limitation. The materials are copper oxide deposited on copper and nickel oxide deposited on nickel conversion coatings. The copper oxide exhibits satisfactory absorption and low emissivity. The nickel coating has a texture which provides antireflection and the nickel oxide provides absorption.
The limitation shared by these two, which is also found in any of the above mentioned selective absorbers, becomes apparent when attempting to employ a selective absorber as discussed above in one of the most significant applications for photothermal devices. That application includes use of a solar concentrator. The solar concentrator functions to focus sunlight onto the selective absorber to concentrate solar radiation. The problem is that when the surfaces sit in air, the elevated temperatures encountered induce oxidation of the materials rendering them substantially useless for their intended purpose.
In evacuated tube-type systems, oxidation is not a problem. However, a further source of inefficiency becomes apparent when a selective absorber made in solution is used in conjunction with these types of collectors. These collectors are generally constructed of two glass tubes, one having a smaller diameter than and being longitudinally arranged within the other. The space between the tubes is evacuated to prevent heat transfer to the outside. The selective absorber material covers the outer surface of the inner tube. In such a configuration, light will penetrate the outer tube and be absorbed by the selective absorber while, theoretically, emission of the heat absorbed is blocked by the evacuated space. The problem arises from the fact that materials made in solution contain water. Materials made in solution out gas water and other gasses into the space between the tubes. The outgassing causes the loss of the vacuum which in turn allows loss through heat transfer.
The device of the present invention utilizes a selective absorber layer of an amorphous material which provides a number of advantages over the prior art materials.
First, the absorption edge of an amorphous material can be changed at will. In other words, the material can be adjusted for maximum absorption over the TSS and minimum emissivity of the TIR wavelengths.
Another advantage over the prior art is that amorphous materials of the present invention can be produced which exhibit greater absorption of radiant energy than crystalline materials having the same composition.
Another advantage is that the index of refraction of an amorphous material can be graded more uniformly from one extreme to the other than can crystalline materials. This is desirable because a gradual increase, as a function of film depth, in the index of refraction causes light to be more readily absorbed. Abrupt changes in the index of refraction contribute to greatly increased reflectivity and therefore decreased absorption.
Amorphous materials can be produced with controlled and predetermined indices of refraction not possible with crystalline and polycrystalline materials of the same composition. This allows matching of the indices of refraction of two adjoining materials to decrease the reflection off the interface of the materials and therefore increase absorption.
Another advantage of the use of amorphous materials in selective absorbers is that amorphous films lack polycrystalline grain boundaries. This is desirable because such grain boundaries act as oxygen or moisture pathways. Oxygen and moisture are two of the predominate causes of film degradation.
It should be noted that a number of alloys can be formed with amorphous materials which do not even exist in crystalline or polycrystalline form. The inherent advantage in the increased number of alloys available for general or specialized applications is obvious.
Finally, amorphous materials are easier and less expensive to deposit and can be deposited at temperatures lower than those required for depositing crystalline or polycrystalline materials. Lower deposition temperatures lead to substantially decreased damage to surfaces on which the material is deposited which in turn allows a substantially greater number of possible substrates to be utilized with the device.